PET has been widely used in clinical examination and disease diagnosis in recent years. A commercial PET scanner currently available typically has a relatively small axial length (e.g., usually 20-30 cm), which usually permits imaging of only a portion of a subject (e.g., a patient) at one time. Recently, PET devices with a long axis length (e.g., 1 meter, 2 meters) have been developed to facilitate the acquisition of PET data. A PET device with a long axial length has a relative wider axial scanning range, providing more PET data than a commercial PET scanner currently available under the same scanning conditions. However, the composition of the PET data acquired by the PET device with a long axial length may be different from that acquired by a commercial PET scanner. For example, the proportion of random event data or the proportion of multiple scattering event data in the entire PET data may be different. Traditional techniques of processing and correcting PET data may be unsuitable for the PET data acquired by a PET device with a long axial length. Thus, it is desirable to provide effective mechanisms for processing and correcting PET data acquired by the PET device with a long axial length.